The present invention relates to optical imaging and, more particularly, to a mask for insertion in the pupil of an optical imaging system that provides extended depth of field for imaging scenes located in the extended depth of field region and illuminated by incoherent light without needing to process the image created by the imaging system that incorporates the mask.
Imaging systems are known to require accurate focus alignment. Conventional imaging systems, for example cameras, are very sensitive to misfocus. When the object and image planes are not in conjugate positions, the resultant image is severely degraded. Nevertheless, there are many applications (e.g. barcode reading, computer or machine vision, surveillance cameras, etc.) that require imaging of objects located anywhere within an extended depth of field (DOF) region, while allowing reduced contrast and resolution.
Conventional solutions for imaging systems with an extended DOF involve pupil stopping and apodization. See, for example, M. Mino and Y. Okano, “Improvement in the OTF of a defocused optical system through the use of shaded apertures”, Applied Optics vol. 10 pp. 2219-2225 (1971); J. O. Castaneda et al., “arbitrary high focal depth with a quasioptimum real and positive transmittance apodizer”, Applied Optics vol. 28 pp. 2666-2669 (1989); J. O. Castaneda and L. R. Berriel-Valdos, “Zone plate for arbitrary high focal depth”, Applied Optics vol. 29 pp. 994-997 (1990). The main disadvantages with such solutions are reduced resolution and low light throughput.
Recently, a hybrid opto-electronic approach has been proposed, to solve such problems. See, for example, E. R. Dowski, Jr. and W. T. Cathey, “Extended depth-of-field through wave-front coding”, Applied Optics vol. 34 (1995) pp. 1859-1866, and J. van der Gracht et al., “Broadband behavior of an optical-digital focus-invariant system”, Optics Letters vol. 21 no. 13 (1996) pp. 919-921. Both of these references are incorporated by reference for all purposes as if fully set forth herein. In that approach, a non-absorptive phase mask is used to severely aberrate, or encode, the wavefront of the light wave at the pupil. The aberration distorts the obtained image (sometimes referred to as the “intermediate image”), often so much that the intermediate image is unidentifiable. Nevertheless, the intermediate image is insensitive to misfocus for a wide range of DOF. Image acquisition is followed by a digital signal-processing (DSP) step to recover the final image from the intermediate image. Another approach, by W. Chi and N. George (“Computational imaging with the logarithmic asphere: theory”, Journal of the Optical Society of America vol. 20 pp. 2260-2273 (2003)) produces distorted images that change with misfocus position. An iterative filter is used in this case to restore the image so as to finally provide imaging with a certain enhanced depth of field. The main advantages of such approaches are that there is no reduction in light power collection, and that theoretically, one can restore the image details up to the optical cutoff spatial frequency. Practically, resolution is limited by CCD pixel size, as well as by the presence of noise, which may even be amplified in the processing stage.
We have proposed a phase mask that consists of sixteen spatially multiplexed Fresnel lenses. See E. Ben-Eliezer et al., “All-optical extended depth of field imaging system”, Journal of Optics A: Pure and Applied Optics vol. 5 (2003) pp. S164-S169, which reference is incorporated by reference for all purposes as if fully set forth herein. Although the corresponding all-optical imaging system has an extended DOF, this imaging system is suboptimal for scenes illuminated by incoherent light.
There is thus a widely recognized need for, and it would be highly advantageous to have, a mechanism, for extending the DOF, such mechanism being optimized for incoherent illumination and not requiring postprocessing.